This invention relates to the electrodeposition of chromium of selectively constituted crystalline character directly on metal substrates and particularly to the fabrication of improved aluminum and steel base lithographic sheet having fine secondary grained chromium directly plated thereon that can operatively function as a surface plate after exposure of an applied photo sensitive coating thereon.
Bi-metal and tri-metal lithographic plates have long been employed as an alternative to deep etch plates in the lithographic arts. Among the multimetal layered lithographic plates that have been commercially employed are the IPI tri-metal plate formed of a steel or zinc base sheet having an intermediate layer of plated copper and a surface layer of chromium plated over the copper; the "Lithure" plate formed initially of a copper sheet plated with chromium and, more recently, of an aluminum base sheet having an intermediate layer of plated copper and a surface layer of chromium plated thereon; the "Aller" plate formed of a stainless steel base plate overlaid with plated copper; and the "Lithengrave" plate comprising a copper plated aluminum base sheet. For the purposes of convenience both 1000 series aluminum sheet, such as 1100, and other aluminum alloy sheet used for lithographic plates, such as 3000 series sheet will be hereinafter termed generally as "aluminum" sheet or "aluminum base" sheet.
In a similar manner steel sheet whether of mild or low carbon steel or of stainless steel will be hereinafter termed generally as "steel" sheet or "steel base" sheet.
The use of steel as a basal sheet substrate for lithograph plate because of its mechanical strength and resistance to cracking on printing presses has long been recognized. As indicated above however such steel base substrates are usually provided with an intermediate coating or layer of another metal, usually copper, between the steel substrate and the electrodeposited chromium.
Although chromium has long been recognized as a preferred surface metal for lithographic sheet and aluminum has long been recognized as a convenient and relatively inexpensive basal sheet substrate therefor, the direct plating of chromium on aluminum base sheet has been a long-sought but hitherto unattainable objective in the lithographic art. The patented art is replete with disclosures delineating the difficulties of directly plating chromium on aluminum or aluminum base substrates and the practical necessity of the incorporation of an intermediate coating therebetween. Whether such difficulties are attributable to the rapidity of oxide formation on aluminum surfaces or are attributable to a basic incompatability between the surface or plating adhesion characteristics of aluminum and chromium, the practical art has always had to employ an intermediate coating, most usually of another metal, such as zinc, or flash coatings, such as copper, to effectively modify the aluminum base surface characteristics to permit chromium to be plated thereon.
The invention may be briefly and broadly described as an improved process for electrodepositing chromium of selectively constituted crystalline character and grain texture directly on aluminum and steel base substrates. In its narrower aspects, the invention may be described as an improved aluminum or steel base bi-metal lithographic plate having a fine secondary grained and interfacially adherent directly plated chromium surface of selectively constituted crystalline character and fine secondary grain texture formed of progressively agglomerated spheroids, and to processes for forming such lithographic plates from aluminum and steel base substrates. In a still further aspect, the invention includes an improved interfacial adherence between such electrodeposited chromium layer and an overlying coating of photo sensitive material.
Among the advantages of the subject invention is the provision of directly electrodeposited chromium surface layers of fine secondary grained, closely adherent character that are characterized by a selectively constituted crystal structure and grain texture on base metal substrates such as aluminum and steel. Other advantages include the provision of a chromium-surfaced aluminum base lithograph plate that operatively functions, after exposure of an applied coating of photo sensitive material, as a surface plate; that is possessed of increased press life in terms of permitted impressions per plate, together with improved abrasion resistance, corrosion resistance, durability and greater resistance to plate cracking. Still further advantages of the subject invention are the provision of a chromium surfaced bi-metal lithographic plate that operatively functions as a surface plate and that is markedly superior in photo sensitive coating adhesion, water carrying ability and tolerance for fountain solutions of varying pH. Additional advantages include increased efficiency of chromium plating and provision of a fine secondary grained and closely adherent directly plated chromium surface layer for lithographic plates of markedly improved character, which provide increased latitude for operator error when using press chemicals and abrasive ink pigments.
The primary object of this invention is the provision of a directly plated chrome surface layer for aluminum and steel base lithograph plate that is operably functional, after exposure, as a surface plate.
Another primary object of this invention is the provision of improved chromium plated surfaces characterized by a selectively constituted crystal structure and grain texture and plating processes to obtain the same.
Another object of this invention is provision of improved aluminum and steel base lithographic plate having a chromium layer directly plated on the surface thereof.
Another object of this invention is the provision of aluminum and steel base lithograph plate having a selectively constituted, fine secondary grained and closely adherent coating of directly deposited chromium on the surface thereof.
Still another object of this invention is the provision of directly chromium plated aluminum and steel base lithographic plate that is operable as a surface plate and which is markedly superior in photo sensitive coating adhesion, water carrying ability, corrosion and mechanical wear resistance.